High-temperature coating processes have the disadvantage that tenacity losses in the composite body consisting of a substrate body and the thereto applied coating of hard substance cannot always be avoided. So, for instance in the case of indexable inserts coated with titanium carbide used in the continuous machining of high-alloy steel, generally a higher degree of wear has been found when the coating was applied by a high-temperature process at approximately 1000.degree.. However at sufficiently low temperatures, such negative effects of the coating process on the substrate body can be avoided. Among the coating processes which can be carried out at low temperatures, besides the PVD process, the plasma-activated CVD process is important. If on the reaction gas in a low-pressure corona effect an unbalanced plasma is superimposed, the charged particles are accelerated under the influence of the existing electric field. Depending on particle density or pressure, the free path length between two impacts is also determined. If the particle energy is sufficient at the applied voltage, molecules or atoms can be excited for dissociation or ionization. In this way chemical reactions become possible which otherwise take place only at relatively high temperatures. The low-pressure plasma can basically be produced by applying a constant direct voltage at a workpiece connected as a cathode, by a high-frequency alternating current or by a pulsed direct voltage.
The high-frequency excitation, wherein the energy can be introduced inductively or capacitively from the outside into the reaction vessel, is often used for the deposition of very pure layers, e.g. in microchips. Since it works with electrodes which are not directly connected with the substrate, it does not matter whether the material itself is conductive or nonconductive. The drawback is that this process is very expensive.
The simplest way to produce a low-pressure discharge is to connect the workpiece as a cathode and to use the vessel or its walls as the anode or ground potential. The temperature of the substrate is thereby a function of the voltage and the current.
Further the direct voltage can be pulsed in which case the substrate temperature is a function of the peak voltage, as well as the peak current and also of the pulse duration and pulse frequency. The advantage of this process is that the coating temperature can be set independently of the parameters of the low-pressure discharge, the voltage and the current.
Basically CVD reactors have to meet two requirements: on the one hand they have to allow the coating of as many substrates as possible, which means they must have a high capacity, on the other hand shading effects or irregular coatings have to be avoided.
With regard to the second above-mentioned requirement it has already been proposed in DE 22 51 571 C3 to circulate the reactive gases in the reactor from the entire marginal area of the substrate support inwards over the substrate support and the substrate bodies supported thereon, from where the gases are then centrally evacuated. In this way a laminar radial flow is meant to be achieved. The reactor has a cover plate which is connected as a first electrode with respect to the substrate receiving plate which is connected as a second electrode. A high-frequency voltage is used for the production of plasma.
In U.S. Pat. No. 4,909,183 at first a dischargeable chamber is described, in whose center a rotatable mounting support or the coating substrate is arranged. The substrate itself is surrounded by a cylindrical cathodic electrode, which is connected with a high-frequency voltage source. The substrate serves as anode, so that when the voltage is applied a low-pressure plasma is produced at the cathode. The disadvantage is that the coating material is deposited not only on the substrate, but also on the cathode, as well as on other walls enclosing the plasma existing in the area. In this case large gas losses result. Finally it is also disadvantageous that sometimes from the electrodes further walls in the reactor vessel, coating materials deposited there break off and fall on the workpiece, where they cause undesired defects.
In order to find a remedy against the above disadvantage in U.S. Pat. No. 4,909,183 is proposed to arrange the anodically connected substrate bodies next to each other on a circular track at a distance from the reactor wall and, in the center of the reactor, to arrange cathodes connected with the high-frequency source. The reaction gas is supposed to flow in through an inlet opening in the reactor shell. The substrate bodies are located on a multitude of rotary drives which can insure a satisfactory coating of the substrates.
U.S. Pat. No. 4,991,542 also describes a reaction chamber in whose center a substrate body connected to a high-frequency voltage source is arranged, on whose sides diametrically opposed electrodes are provided, which at the same time serve as gas-supply devices. The electrodes are funnel-shaped and have on the side facing the substrate several gas outlet openings in the approximate shape of a shower head. Instead of connecting the voltage source with the substrate support, these can also be connected with grid electrodes, each arranged on both sides between the substrate and the shower-head electrode.
The EP-A-0 149 408 describes a device for low-temperature coating with a housing having a cage at its inner shell, which is supposed to enclose magnetically the plasma formed inside the housing. A glow filament arranged in the inner cage space serves to excite the plasma; the walls of the housing represent an electrode for producing plasma.
The DE-A-34 17 192 describes a device for forming an amorphous silicon film on a substrate. Between a substrate body and a counterelectrode a plasma is produced, which is surrounded by a cylindrical net structure. The latter is supposed to prevent radicals from expanding outside the net structure. The net structure is supposed to be arranged at a distance from the circumference of the object to be coated which is shorter than the average free path length of the electrons present in the plasma production area.
However, all heretofore-described processes, respectively devices, could yield only unsatisfactory results regarding the coating uniformity of the substrate bodies.